The alkaline earth metal salts of 2-pyrrolidone-5-carboxylic acid are of considerable interest. Thus, for example, magnesium pyroglutamate is used as a pharmaceutical preparation in the treatment of magnesium-deficiency diseases (see Published French Patent Application FR-OS 2,546,064). Calcium pyroglutamate has proven to be an especially useful agent for the replacement of calcium in bone tissue in the case of acute calcium deficiency [E. Moczar, B. Phan Dinh Tuy, L. Robert; "Rhumatologie", Paris, 8, p. 71, (1979)]. Calcium pyroglutamate is also used as corrosion inhibitor in projectile explosives ["Gor. Rep. Announce. Index", USA, 79, p. 227, (1979)].
However, in the past the synthesis of L-calcium pyroglutamates were relatively complicated. For example, the cyclization of a mixture of L-glutamic acid and calcium oxide in aqueous solution at 140.degree. C. and elevated pressure has been described (ES-PS 528,010). A disadvantage of this process is that only an aqueous solution of the calcium pyroglutamate is obtained and that a complete racemization to D,L product occurs. The product used in the pharmaceutical preparations contains only the L-enantiomer, since only the latter is physiologically active.
Another possibility for the preparation of this compound is to react L-pyroglutamic acid with alkaline earth metal hydroxides, oxides or carbonates in water (see published French Patent Application FR-OS 2,546,064); however, only an aqueous solution of the salt is produced in this case as well. Moreover, this process requires the use of L-pyroglutamic acid as a starting material. Pyroglutamic acid is formed by heating L-glutamic acid in concentrated aqueous solution, but the solution is racemized and an equilibrium forms between the starting material and the final product. A further separation step is required for the synthesis of pure L-pyroglutamic acid [see P. M. Hardy, "Synthesis", 1978, p. 290]. The direct heating of L-glutamic acid in the melt to 180.degree. to 185.degree. C. is also accompanied by racemization and, especially, by very strong decomposition [N. Lichtenstein, N. Gertner, "J. Am. Chem. Soc." 64, p. 1021, (1946)].
Finally, the reaction of calcium pyroglutamate with magnesium sulfate has been described for the preparation of magnesium pyroglutamate (French Patent FR-PS M 3593). However, this leaves the problems of synthesis of calcium pyroglutamate which have been described above.